Conventional fire-fighting equipment mixes a secondary fluid (such as a foaming agent) with a primary fluid (such as water) for extinguishment of certain types of fires. It is important that a precise, predetermined proportion (percentage) of foaming agent be added to the water because the foaming agent/water combination is most effective for extinguishment when an exact proportion is used, and also because using excess foaming agent is wasteful and expensive.
The foaming agent is usually added to the water by an eductor (Venturi) placed in a branched circuit flowing from the outlet of a water pump to the inlet of the pump. The water flows through the eductor, creating a suction (by Venturi action) which pulls in the foaming agent through a side inlet of the eductor, with the water and foaming agent thereafter flowing and mixing together. The eductor is basically a constant percentage pick-up device and requires a variable metering (proportioning) device between the foaming agent storage tank and the eductor, if the discharge flow rate varies, e.g. as when a plurality of fire-fighting nozzles supplied simultaneously are individually turned on and off during use. It can be difficult to maintain the proper proportion of foaming agent using a variable resistance passage. For the proportion of foaming agent-to-water to remain constant, the passage (metering) opening must be continually reset as the rate of fluid flow through the system changes due to the nozzles being cut in and out during the fire-fighting operation.
U.S. Pat. No. 4,503,915 describes a proportioning valve for use in fire-fighting equipment which automatically regulates the introduction of secondary fluid into a primary fluid flow. This valve contains a plurality of flow regulating passages (orifices), each being sized to accommodate a particular piece of fire-fighting equipment. for a particular percentage of foaming agent. The proper orifice is automatically opened and closed for a particular nozzle when that nozzle is discharged or shut down. If a foaming agent requiring a different percentage is to be used, the metering valve must be disassembled and rebuilt with a replacement component having the new required orifices. This system was a prior advance in this field by the present inventor, and others, over which the present invention represents a further improvement.